This is a multi-center, randomized, double-blind, placebo-controlled clinical trial in nondemented women, 65 years of age or older, with a family history of AD to determine if estrogen (alone or combined with progesterone as appropriate)can delay the onset of Alzheimer's disease and prevent memory decline. At study entry, family history of AD will be confirmed and each participant will have a physical, neurological, neuropsychological and functional assessment to insure the absence of dementia, any other degenerative neurological disease or potentially fatal disorder, rule out a history of breast, uterine or ovarian cancer and those with a history of breast cancer in a first-degree relative. Randomization to estrogen, estrogen with progesterone, or identical placebo among eligible women will be stratified by site and hysterectomy status (hysterectomized women will be randomized to unopposed estrogen or placebo; nonhysterectomized women to opposed estrogen or placebo). Patients will be followed over a 3 year (36 months) period, and will be examined at 6 month intervals to assess compliance, adverse events and general health status. Annual complete medical, gynecological, neuropsychological and functional assessments will occur during follow-up. Outcome measures will include incident dementia and memory decline and will use an intent-to-treat analysis. APOE genotype, educational level and ethnic group are potential covariates for the secondary analyses. Safety evaluations will be based on findings from annual assessments and reported adverse events.